Electroluminescent phosphors, particularly those based on zinc sulfide, and the methods of manufacturing the same, are known. The most common types emit in the blue and green portions of the spectrum. The green emitting phosphors have a longer half-life (i.e., the time at which the brightness is one half the initial brightness), typically in the range of about 10-1000 hours when operated at 100 volts at 400 Hz. The half-life is shortened when the lamps are operated at higher voltages, frequencies or temperatures.
Typically, zinc sulfide electroluminescent phosphors have been prepared by preparing a mixture of predominantly ZnS with about one weight percent (wt. %) of a copper compound such as CuSO.sub.4, and a few wt. % of a halide co-activator as a fluxing compound. This mixture was then fired at a temperature of from 1000 to 1200.degree. C. for 4 to 6 hours and, after cooling, washed to remove the flux. The washed material was then stressed (for example, by milling or mulling) and then refired at 600 to 950.degree. C.
When a thusly-prepared phosphor was subsequently fired in vacuo at a temperature of 500.degree. C. for 72 hours the half-life was determined to be 390 hours.
It would be an advance in the art if the half-life of electroluminescent phosphors could be extended.